The present invention relates to a compound lens for use in arrays of lenses and in particular to a compound lens for use in arrays of lenses or for use in projectors arranged in an array in multi-perspective autostereo projection systems.
In known multi-perspective autostereo projection systems the images from a plurality of projectors which show different perspective views of an object are projected onto a direction selective projection screen, such as a field lens. Such screens have the characteristic of re-forming the array of images of the projector lens exit pupils in a xe2x80x9cviewer spacexe2x80x9d so that each viewer of the screen sees a stereo image pair and accordingly sees a three dimensional image. Thus, if a viewer horizontally crosses between adjacent images, the three dimensional image changes discontinuously or xe2x80x9cflipsxe2x80x9d. A problem with this type of display is that the images seen by the viewer are separated by dark regions associated with gaps between the projection lenses.
In a paper entitled xe2x80x9cAutostereoscopic 3D-imaging by front and rear projection and on flat panel displaysxe2x80x9d by R Borner which was published in xe2x80x9cDisplaysxe2x80x9d, Volume 14, Number 1, 1993 by Butterworth-Heinemann Ltd, this problem is tackled by overlapping the exit pupils of the projector lenses by using at least two vertically spaced layers of projectors lenses horizontally off-set with respect to each other. In this way the axis of an exit pupil of a projector lens in a first layer will lie in a horizontal plane located between the axes of the exit pupils of two adjacent projector lenses in a second layer. The images of the projector lens exit pupils from different layers can be overlapped in the xe2x80x9cviewer spacexe2x80x9d using lenticular screens in order to eradicate dark regions between the images in the xe2x80x9cviewer spacexe2x80x9d. A similar approach is used in a paper entitled xe2x80x9cAn autostereoscopic real-time 3D display systemxe2x80x9d by G. Bader, E. Luederand J. Fuhmann published in Euro Display ""96. The resulting display apparatus using such multi-layer arrangements can be overly complex and the use of lenticular arrays to spread out the exit pupil image can affect the projected picture quality.
In a paper entitled xe2x80x9cMultiperspective autostereoscopic displayxe2x80x9d by Gordon R. Little, Steven C. Gustafson and Vasiliki E. Nikolaou published in SPIE Volume 2219 Cockpit Displays (1994), the problem of gaps between adjacent images in the xe2x80x9cviewer spacexe2x80x9d is solved by using a pupil forming screen which comprises a Fresnel lens and a lenticular array to spread out each image in the xe2x80x9cviewer spacexe2x80x9d to remove any gaps between adjacent images. This again has the disadvantage of affecting the projected picture quality by reducing the resolution of the display.
In WO98/43441 an autostereo projection system is provided which uses multiple projectors to form an essentially seamless and extended field of view. Each projector is registered to a CRT display and shutter elements in each projector create multiple pupils for each CRT display. Each CRT display is driven with different images for each shutter pupil. Seamlessness is promoted by the use of a tunable optical diffuser as part of a common lenticular viewing lens. In WO98/43441 the adjacent compound lenses of the projectors abut, however, the exit pupils of the compound lenses do not abut, which results in dark spaces between adjacent images in the viewer space which are reduced by the use of the diffuser.
The present invention aims to overcome at least some of the problems discussed above by providing a compound lens for use in arrays of lenses which substantially eliminates the problem of dark spaces between images in the xe2x80x9cviewer spacexe2x80x9d without requiring over-complex display arrangements and without reducing the resolution of the image seen by the viewer.
According to a first aspect of the present invention there is provided a compound lens for use in an array of such lenses comprising at least two lens elements including a front lens element having a front lens surface which is the largest diameter lens surface in the compound lens, wherein the exit pupil of the compound lens is bounded by and lies in the plane of the edge of said front lens surface. In such a compound lens the exit pupil lies at the front of the compound lens at the largest diameter lens surface of the compound lens which means that in an array of such compound lenses, adjacent abutting lenses will have adjacent abutting exit pupils. Thus, there will be no gaps between the exit pupils of adjacent compound lenses and so the present invention can be used to eliminate the problem of dark spaces between images in the xe2x80x9cviewer spacexe2x80x9d in autostereo projection systems. The compound lens according to the present invention is of use in any application requiring the use of arrays of lenses with abutting exit pupils.
The aperture stop may be located within the front lens element of the compound lens. Preferably, the aperture stop of the lens lies in front of the lens element(s) of the compound lens other than the front lens element as this improves the symmetry of the lens arrangement about the aperture stop and helps to reduce coma, distortion and transverse colour in the compound lens. The aperture stop of the compound lens may lie in a plane which intersects the optical axis of the compound lens at the rear lens surface of the front lens element.
The front lens element may itself be a compound lens or alternatively could comprise a single lens. It is preferred that the rear lens surface of the front lens element is concave and further that the front lens surface of the front lens element is convex.
In a preferred arrangement the front lens element is the largest diameter lens element in the compound lens.
The first aspect of the present invention relates to a compound lens with its exit pupil located at its last (or front) surface with the edge of the exit pupil being the intersecting ring of the plane where the exit pupil is located and said last surface. The diameter of the exit pupil defines the largest diameter of the compound lens and the radius of the exit pupil is larger than or equal to all the ray heights traced through the compound lens without vignetting. Thus, several such compound lenses can be arranged in an array so that their exit pupils abut so that there will be no dark zone when this compound lens array is used as the projection lens array in a multi-projector autostereoscopic display.
According to a second aspect of the present invention there is provided a projector for use in an array of such projectors comprising a compound lens according to the first aspect of the present invention. As discussed above such a projector can be used in an abutting array of such projectors in an autostereo projection system in order to eliminate gaps between images in the xe2x80x9cviewing spacexe2x80x9d of the system.
According to a third aspect of the present invention there is provided an autostereo projection system comprising an array of projectors according to the second aspect of the present invention.
According to a fourth aspect of the present invention there is provided a method of designing a compound lens for use in an array of such lenses comprising the steps of;
defining the material of a front lens element of the compound lens, the diameter of a front lens surface of the front lens element, the radius of curvature of the front lens surface and a rear lens surface of the front lens element and defining the location of an exit pupil of the compound lens arrangement to be bounded by and in the plane of the edge of said front lens surface,
based on the above defined parameters, tracing the location and magnitude of an aperture stop of the compound lens by tracing the marginal rays through the front lens element using ray tracing means,
repeating the above steps until the marginal ray heights through the front lens element are highest at the exit pupil, and then fixing the above defined parameters,
defining the diameters of the remaining lens surfaces of the compound lens to be less than that of the front lens surface of the front lens element and defining the functionality of the compound lens,
using ray tracing means to design the remainder of the compound lens, in such a way that the remaining lens surfaces do not alter the relationship between the exit pupil and the marginal ray height through the front lens element defined above.
Preferably, the parameters defined in the first step of the method are only fixed when the diameter of the aperture stop is less than the diameter of the exit pupil.
This method can be used to design a compound lens according to the first aspect of the present invention and which will have the advantages associated with the first aspect of the present invention.
The term marginal ray is used generally to describe those rays which pass through the edge of an aperture stop or an entrance or exit pupil of a lens system. The marginal ray is that ray which is the highest (ie. moves furthest away from the optical axis) amongst the rays traced through a lens system from an object point. In a compound lens according to the present invention the marginal rays will pass through the edge of the exit pupil, because the exit pupil is real.
Preferably, the step of defining the diameter of the front lens surface of the front lens element comprises the step of defining the diameter of the front lens element.
Preferably, the step of defining the functionality of the compound lens comprises the step of defining the compound lens as a finite conjugate lens with specified object and image distances as such a compound lens would be suitable for use in a projector of an autostereo projection system. Alternatively, the compound lens could be defined as a finite-infinite conjugate lens element
For the purposes of the present invention the aperture stop shall be defined as the image of the exit pupil.